Ultrahigh-molecular-weight polyolefins, for example ultrahigh-molecular-weight polyethylene, have higher impact strength, abrasion resistance, chemical resistance, tensile strength, etc. than general-purpose polyolefins such as general-purpose polyethylene, and have found increasing applications as engineering plastics. The ultrahigh-molecular-weight polyethylene, however, has the defect that is is very difficult to mold by extrusion or injection-molding because it has a much higher melt viscosity and thus lower flowability than general-purpose polyethylene.
At present, therefore, most articles from ultrahigh-molecular-weight polyethylene are produced by compression molding, and only some articles such as rods are produced by extrusion-molding at very low speeds.
When such ultrahigh-molecular-weight polyethylene having low melt-flowability is molded by an ordinary injection-molding method, shear broken streams of the polymer always form during the filling of the polymer in a mold cavity, and the resulting molded article undergoes delamination like mica and cannot exhibit the excellent properties of the ultrahigh-molecular-weight polyethylene. Rather, its quality is inferior to that of a molded article of general-purpose polyethylene.
Japanese Patent Publication Nos. 30067/1982 and 58010/1985 propose a injection-molding method free from causing delamination in which the capacity of a mold cavity is slightly increased before or after a resin is filled in the cavity, and then the resin is compressed to a predetermined volume. This method enabled production of injection-molded articles free from delamination and having the impact strength and abrasion resistance which are the inherent characteristics of the ultrahigh-molecular-weight polyethylene. To perform injection molding by this method, however, it is necessary to use an injection-molding machine equipped with a variable mold cavity system, and the general injection-molding machine for polyethylene cannot be used as it is.
On the other hand, for improving the melt-flowability of ultrahigh-molecular-weight polyolefins, mixing of an ultrahigh-molecular-weight polyolefin with a low to a high molecular weight polyolefin was proposed.
Japanese Patent Publication No. 27,064/1971 discloses an abrasion-resistant polyethylene resin composition comprising polyethylene having an average molecular weight of at least 500,000 and 20 to 50% by weight of polyethylene having a density of at least 0.940 and an average molecular weight of 30,000 to 120,000.
Japanese Patent Publication No. 30,293/1972 discloses a process for producing a material for use in that surface of agricultural machines, earth-moving machines, etc. which comes into contact with the soil, which comprises mixing screw-extrudable polyethylene having a molecular weight of not more than 200,000 and produced by the medium-pressure or the low-pressure method with 10 to 30% by weight of ultrahigh-molecular-weight polyethylene having a molecular weight of about 1 million and being incapable of screw extrusion, melting the mixture uniformly, and continuously molding the uniform molten mixture by an extruder.
Japanese Patent Publication No. 41,309/1983 discloses a polyethylene composition comprising a mixture of 85 to 50 parts by weight of polyethylene having a viscosity average molecular weight of 500,000 to 150,000 and 15 to 50 parts by weight of granular ultrahigh-molecular-weight polyethylene having a viscosity average molecular weight of at least 1 million and a particle size smaller than 10 mesh.
Japanese Laid-Open Patent Publication No. 177,036/1982 discloses an ultrahigh-molecular-weight polyethylene composition having improved moldability comprising 100 parts by weight of ultrahigh-molecular-weight polyethylene having a molecular weight of at least 1 million and 10 to 60 parts by weight of low-molecular-weight polyethylene having a molecular weight of 5,000 to 20,000. The specification of this patent document states that the moldability of this ultrahigh-molecular-weight polyethylene composition is such that in the production of a slab having a thickness of 50 mm by a compression molding method, the molding cycle required was decreased to 200.degree. C..times.2 hours while with the ultrahigh-molecular-weight polyethylene alone a molding cycle of 200.degree. C..times.3 hours was required; and that in a ram extrusion method, the speed of pipe extrusion was increased from 5 cm/min. to 10 cm/min.
Japanese Laid-Open Patent Publication No. 126,446/1984 discloses an ultrahigh-molecular-weight polyethylene resin composition comprising 95 to 50 parts by weight of an ultrahigh-molecular-weight polyethylene resin and 5 to 50 parts by weight of a general-purpose polyolefin resin. The specification of this document discloses a composition in which a silane-modified polyethylene resin having a melt index of 2.5 or 5.0 g/10 min. is used as an actual specific example of the general-purpose polyolefin resin.
Japanese Patent Publication No. 41,309/1983 discloses a polyethylene composition comprising a mixture of 85 to 50 parts by weight of polyethylene having a viscosity average molecular-weight of 500,000 to 150,000 and 15 to 50 parts by weight of granular ultrahigh-molecular-weight polyethylene having a viscosity average molecular weight of at least 1 million and a particle size smaller than 10 mesh. As stated in column 3, lines 17-28 of this document, the moldability of the ultrahigh-molecular-weight polyethylene in this composition has not improved, but the purpose of providing this composition is to produce a molded article having excellent impact strength by reducing anisotropy utilizing the granular state of the ultrahigh-molecular-weight polyethylene.
The above polyethylene resin compositions are prepared by mixing ultrahigh-molecular-weight polyethylene with polyethylene or other polyolefins having lower molecular weights.
Japanese Laid-Open Patent Publication No. 94,593/1979 (corresponding to U.S. Pat. No. 4,414,369) discloses a process for producing a polyolefin having a broad molecular weight distribution by polymerizing an olefin in the presence of a solvent and hydrogen using a Ziegler-type catalyst of enhanced activity supported on a solid carrier using a plurality of reactors, to produce a polyolefin continuously, which comprises continuously feeding a main olefin monomer and at least one olefin comonomer under pressure to the reactor in which a gaseous phase containing an inert gas is present in the upper part of a first-stage reactor, and copolymerizing these monomers, continuously transferring the polymerization reaction mixture in which high-molecular-weight polymer particles are dispersed in a solvent to a second-stage reactor composed of a vertical stirred vessel maintained at a lower pressure than the first-stage reactor by differential pressures without substantially separating part of the components in the mixture and without using any forced transferring means, continuously performing polymerization in the second-stage stirred vessel in the presence of the main olefin monomer and hydrogen while a gaseous phase exists in the upper part of the stirred vessel, thereby to form a polymer having a lower molecular weight than in the first stage polymerization, continuously withdrawing the polymerization reaction mixture containing the resulting polymer particles dispersed in a solvent from the second-stage stirred vessel, and recovering the polymer.
Japanese Patent Publication No. 10,724/1984 (corresponding to U.S. Pat. No. 4,336,352) discloses a process in which polyethylenes of different molecular weights are produced by multi-stage continuous polymerization in at least three polymerization vessels connected in series. The purpose of this process is to produce polyethylene having excellent properties and moldability with high productivity. With regard to moldability, this process is for producing polyethylene by extrusion molding, above all blow molding, with improved die swelling. It does not relate to an improvement in an injection-molded article. Even when a composition containing not more than 10% by weight of ultrahigh-molecular-weight polyethylene and having an MI of 0.3 or an intrinsic viscosity [.eta.] of 2.3 to 3.0 dl/g (corresponding to an MI of about 0.2 to 0.8) is used in injection molding, the content of ultrahigh-molecular-weight polyethylene is as low as not more than 10% by weight.
Japanese Patent Publication No. 11,349/1971 discloses a process which comprises producing 5 to 30% by weight of an ethylene/alpha-olefin copolymer having a reduced specific viscosity of 30 to 5 in a first step, and producing polyethylene or an ethylene/alpha-olefin copolymer having a reduced specific viscosity of 4.6 to 1.5 in a second stage to obtain a homogeneous mixture of it with the polymer obtained in the first stage. The purpose of this process is to improve moldability in the extrusion molding of bottles, cables, pipes, etc. and does not pertain to an improvement in an injection-molded article.
Japanese Laid-Open Patent Publication No. 141,409/1982 discloses a process for producing polyethylene which comprises polymerizing ethylene, or copolymerizing ethylene with an alpha-olefin, using a catalyst comprising a reaction product between a magnesium compound and a titanium halide and an organoaluminum compound; wherein the following three polymerization steps,
(a) a step of forming an ethylene polymer of copolymer having an alpha-olefin content of not more than 10% by weight and an intrinsic viscosity [.eta.] of 0.3 to 1.5,
(b) a step of forming an ethylene polymer or copolymer having an alpha-olefin content of not more than 30% by weight and an intrinsic viscosity [.eta.] of 1.5 to 7, and
(c) a step of forming an ethylene polymer or copolymer having an alpha-olefin content of not more than 30% by weight and an intrinsic viscosity [.eta.] of 7 to 40, are carried out in any desired sequence, and the polymerization reactions are carried out while adjusting the ratio of the amounts of the polymers formed in step (a):step (b):step (c) to 1:0.1-1.5:0.01-1.2.
Japanese Laid-Open Patent Publication No. 8713/1983 discloses a process for producing an ethylenic copolymer which comprises copolymerizing ethylene and an alpha-olefin using a catalyst system obtained from (A) a solid catalyst component containing at least magnesium, halogen and titanium atoms and (B) an organoaluminum compound in at least two steps, wherein
(1) in at least one step, 80 to 20 parts by weight of a copolymer having a high-load melt index of 0.03 to 10 g/10 min. and a density of 0.890 g/cm.sup.3 to less than 0.905 g/cm.sup.3 is produced, (2) in a second step, 20 to 80 parts by weight of a copolymer having a melt index of 10 to 5000 g/min. 10 min. and a density of 0.905 to 0.940 g/cm.sup.3 is produced, whereby a copolymer having a melt index of 0.02 to 30 g/10 min. and a density of 0.890 to 0.935 g/cm.sup.3 is produced.
This patent document discloses that the high-load melt index was measured at a temperature of 190.degree. C. under a load of 21.6 kg in accordance with JIS K-6760.
Japanese Laid-Open Patent Publication No. 8712/1983 discloses a process for producing an ethylenic copolymer using the same catalyst as described in the above-cited Japanese Laid-Open Patent Publication No. 8713/1983 by multi step polymerization, wherein
(1) in at least one step, 80 to 20 parts by weight of a copolymer of ethylene with propylene and/or butene-1 having a highload melt index of 0.03 to 10 g/10 min. and a density of 0.890 to 0.935 g/cm.sup.3 is produced and
(2) in at least one other step, 20 to 80 parts by weight of a copolymer of ethylene and an alpha-olefin having at most 12 carbon atoms as a comonomer having a melt index of 10 to 5000 g/10 min. and a density of 0.890 to 0.940 is produced, said alpha-olefin containing at least 30 mole % of alphaolefins having 5 to 12 carbon atoms, whereby a copolymer having a melt index of 0.02 to 30 g/10 min. and a density of 0.890 to 0.936 g/cm.sup.3 is produced.
Japanese Laid-Open Patent Publication No. 120,605/1984 discloses a process for producing an ultra-high molecular-weight polyethylene resin with improved moldability and processability, which comprises polymerizing monomers using a Ziegler-type catalyst comprising a solid catalyst component containing a trasition metal ingredient and an organometallic catalyst component in at least two steps having different monomer compositions and hydrogen concentrations; wherein in at least one step, propylene or monomers mainly comprising propylene, or butene-1 or monomers mainly comprising butene-1 are polymerized in the presence of hydrogen to produce 2 to 60% by weight, based on the entire polymer to be produced, of a polypropylene or polybutene-1 component, and in at least one remaining step, ethylene or monomers mainly comprisig ethylene are polymerized in the substantial absence of hydrogen to produce 98 to 40% by weight, based on the entire polymer produced, of an ultrahigh-molecular-weight polyethylene component.
British Patent No. 1,174,542 discloses a process for the preparation of a homo- or co-polymer of ethylene by a gaseous phase polymerization, or by a suspension polymerization in which the dispersion medium is in contact with a gaseous phase, of ethylene or a mixture comprising ethylene and up to 10% by weight of an alpha-olefin that contains from 3 to 15 carbon atoms, which process comprises preparing from 5 to 30% by weight of the total polymer in the presence of from 0 to 10% of hydrogen, calculated on the total volume of the gaseous phase, and preparing from 70 to 95% by weight of the total polymer in the presence of from 20 to 80% of hydrogen, calculated on the total volume of the gaseous phase, both stages of the polymerization being carried out at a temperatur within the range of from 50.degree. to 120.degree. C. and a pressure of up to 10 atmospheres gauge, in the presence of a catalyst which is present in the first stage in an amount sufficient for both stages, said catalyst comprising
(a) in the case of a suspension polymerization, from 0.05 to 3.0 millimol per liter of dispersion medium, or in the case of a polymerization in the gaseous phase, from 0.05 to 0.3 millimol per 0.5 liter of reactor volume, of a trivalent titanium compound that contains chlorine, and
(b) from 0.1 to 3.0 millimols of aluminum per liter of dispersion medium or reactor volume, in the form of an aluminum trialkyl having the general formula AlR.sub.3, in which each R represents a hydrocarbon radical that contains from 4 to 40 carbon atoms, or in the form of the reaction product of an aluminum trialkyl or an aluminium alkyl hydride with a diolefin that contains from 4 to 20 carbon atoms.
Japanese Laid-Open Patent Publication No. 275,313/1986 discloses an ultrahigh-molecular-weight polyethylene composition having improved injection-moldability which has an intrinsic viscosity, determined in decalin at 135.degree. C., of 10 to 30 dl/g and obtained by polymerization reaction in at least two steps mentioned below.